Limited supply of fossil energy resources and their associated global environmental damage have compelled market forces to diversify energy resources and related technologies. One such resource that has received significant attention is solar energy, which employs photovoltaic systems to convert light into electricity. Typically, photovoltaic production has been doubling every two years, increasing by an average of 48 percent each year since year 2002, making it the world's fastest-growing energy technology. By midyear 2008, estimates for cumulative global solar energy production stands to at least 12,400 megawatts.
Accordingly, solar concentrators represent promising approaches for mitigating costs associated with photovoltaic (PV) cells. In general, PV concentrators employ low cost materials such as large area glass mirrors to intensify sunlight, and reduce amount of required semiconductor material deemed expensive. In effect, PV concentrators can reduce a dollar-to-watt cost barrier, which typically impedes conventional PV Industry. Moreover, PV concentrators can provide performance advantages, as high cell efficiencies and sun tracking become prevalent.
A significant challenge to achieve increased cost effectiveness is enabling silicon photovoltaic cells to operate efficiently at high intensities while maintaining relatively low manufacturing costs. To meet such challenge, high voltage silicon vertical multi-junction (VMJ) photovoltaic cells have been proposed as an attractive solution. Nonetheless, active layers positioned at ends of such cells are susceptible to damage. In addition, such end layers are more susceptible to mechanical stress caused during manufacturing, and/or to thermal stress induced during high intensity operation resulting from a mismatch of thermal expansion coefficients of contact metals applied thereto (e.g., as electrical leads.) For example, welding of metal contacts to such end layers can adversely affect properties of the active layers, and hence degrade over all performance of the cell. Similarly, intrinsic mechanical stress induced during such fabrication can negatively affect behavior for adjacent underlying diffused junctions.
Likewise, problems can arise due to stress from thermal cycling in high intensity operation, which is caused by differential thermal expansion coefficients of the electrical leads. Such thermal expansion mismatch can further induce thermal strain, which can also affect the underlying active junctions, and degrade overall cell performance or threaten the long-term structural integrity of the VMJ cell.
Such problems are further compounded in cell arrays, wherein a plurality of VMJ cells are connected in series (e.g., via lead contacts), and the poorest performing cell limits operations of the overall array.